Network Working Group C. Jennings, Ed.
Internet-Draft Cisco Systems
Updates: 3261,3327 (if approved) R. Mahy, Ed.
Expires: September 6, 2006 Plantronics
March 5, 2006
Managing Client Initiated Connections in the Session Initiation Protocol
(SIP)
draft-ietf-sip-outbound-02
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Session Initiation Protocol (SIP) allows proxy servers to initiate
TCP connections and send asynchronous UDP datagrams to User Agents in
order to deliver requests. However, many practical considerations,
such as the existence of firewalls and Network Address Translators
(NATs), prevent servers from connecting to User Agents in this way.
Even when a proxy server can open a TCP connection to a User Agent,
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Internet-Draft Client Initiated Connections in SIP March 20061. Introduction
There are many environments for SIP [5] deployments in which the User
Agent (UA) can form a connection to a Registrar or Proxy but in which
the connections in the reverse direction to the UA are not possible.
This can happen for several reasons. Connection to the UA can be
blocked by a firewall device between the UA and the proxy or
registrar, which will only allow new connections in the direction of
the UA to the Proxy. Similarly there may be a NAT, which are only
capable of allowing new connections from the private address side to
the public side. This specification allows SIP registration when the
UA is behind such a firewall or NAT.
Most IP phones and personal computers get their network
configurations dynamically via a protocol such as DHCP (Dynamic Host
Configuration Protocol). These systems typically do not have a
useful name in the Domain Name System (DNS), and they definitely do
not have a long-term, stable DNS name that is appropriate for binding
to a certificate. It is impractical for them to have a certificate
that can be used as a client-side TLS certificate for SIP. However,
these systems can still form TLS connections to a proxy or registrar
which authenticates with a server certificate. The server can
authenticate the UA using a shared secret in a digest challenge over
that TLS connection.
The key idea of this specification is that when a UA sends a REGISTER
request, the proxy can later use this same network "flow"--whether
this is a bidirectional stream of UDP datagrams, a TCP connection, or
an analogous concept of another transport protocol--to forward any
requests that need to go to this UA. For a UA to receive incoming
requests, the UA has to connect to a server. Since the server can't
connect to the UA, the UA has to make sure that a flow is always
active. This requires the UA to detect when a flow fails. Since,
such detection takes time and leaves a window of opportunity for
missed incoming requests, this mechanism allows the UA to use
multiple flows to the proxy or registrar. This mechanism also uses a
keep alive mechanism over each flow so that the UA can detect when a
flow has failed.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [4].
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Internet-Draft Client Initiated Connections in SIP March 20062.1. Definitions
Edge Proxy: An Edge Proxy is any proxy that is located topologically
between the registering User Agent and the registrar.
flow: A Flow is a network protocol layer (layer 4) association
between two hosts that is represented by the network address and
port number of both ends and by the protocol. For TCP, a flow is
equivalent to a TCP connection. For UDP a flow is a bidirectional
stream of datagrams between a single pair of IP addresses and
ports of both peers. With TCP, a flow often has a one to one
correspondence with a single file descriptor in the operating
system.
reg-id: This refers to the value of a new header field parameter
value for the Contact header field. When a UA registers multiple
times, each simultaneous registration gets a unique reg-id value.
instance-id: This specification uses the word instance-id to refer to
the value of the "sip.instance" media feature tag in the Contact
header field. This is a Uniform Resource Name (URN) that uniquely
identifies this specific UA instance.
outbound-proxy-set A configured set of SIP URIs (Uniform Resource
Identifiers) that represents each of the outbound proxies (often
Edge Proxies) with which the UA will attempt to maintain a direct
flow.
3. Overview
Several scenarios in which this technique is useful are discussed
below, including the simple co-located registrar and proxy, a User
Agent desiring multiple connections to a resource (for redundancy for
example), and a system that uses Edge Proxies.
3.1. Summary of Mechanism
The overall approach is fairly simple. Each UA has a unique
instance-id that stays the same for this UA even if the UA reboots or
is power cycled. Each UA can register multiple times over different
connections for the same SIP Address of Record (AOR) to achieve high
reliability. Each registration includes the instance-id for the UA
and a reg-id label that is different for each flow. The registrar
can use the instance-id to recognize that two different registrations
both reach the same UA. The registrar can use the reg-id label to
recognize that a UA is registering after a reboot.
When a proxy goes to route a message to a UA for which it has a
binding, it can use any one of the flows on which a successful
registration has been completed. A failure on a particular flow can
be tried again on an alternate flow. Proxies can determine which
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flows go to the same UA by comparing the instance-id. Proxies can
tell that a flow replaces a previously abandoned flow by looking at
the reg-id.
UAs use the STUN (Simple Traversal of UDP through NATs) protocol as
the keep alive mechanism to keep their flow to the proxy or registrar
alive.
3.2. Single Registrar and UA
In this example, a single server is acting as both a registrar and
proxy.
+-----------+
| Registrar |
| Proxy |
+-----+-----+
|
|
+----+--+
| User |
| Agent |
+-------+
User Agents which form only a single flow continue to register
normally but include the instance-id as described in Section 4.1.
The UA can also include a reg-id parameter is used to allow the
registrar to detect and avoid using invalid contacts when a UA
reboots or reconnects after its old connection has failed for some
reason.
For clarity, here is an example. Bob's UA creates a new TCP flow to
the registrar and sends the following REGISTER request.
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/TCP 192.0.2.1;branch=z9hG4bK-bad0ce-11-1036
Max-Forwards: 70
From: Bob <sip:bob@example.com>;tag=d879h76
To: Bob <sip:bob@example.com>
Call-ID: 8921348ju72je840.204
CSeq: 1 REGISTER
Supported: path
Contact: <sip:line1@192.168.0.2>; reg-id=1;
;+sip.instance="<urn:uuid:00000000-0000-0000-0000-000A95A0E128>"
Content-Length: 0
The registrar challenges this registration to authenticate Bob. When
the registrar adds an entry for this contact under the AOR for Bob,
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the registrar also keeps track of the connection over which it
received this registration.
The registrar saves the instance-id and reg-id along with the rest of
the Contact header field. If the instance-id and reg-id are the same
as a previous registration for the same AOR, the proxy uses the most
recently created registration first. This allows a UA that has
rebooted to replace its previous registration for each flow with
minimal impact on overall system load.
When Alice sends a request to Bob, his proxy selects the target set.
The proxy forwards the request to elements in the target set based on
the proxy's policy. The proxy looks at the target set and uses the
instance-id to understand that two targets both end up routing to the
same UA. When the proxy goes to forward a request to a given target,
it looks and finds the flows that received the registration. The
proxy then forwards the request on that flow instead of trying to
form a new flow to that contact. This allows the proxy to forward a
request to a particular contact over the same flow that the UA used
to register this AOR. If the proxy has multiple flows that all go to
this UA, it can choose any one of registration bindings for this AOR
that has the same instance-id as the selected UA. In general, if two
registrations have the same reg-id and instance-id, the proxy will
favor the most recently registered flow. This is so that if a UA
reboots, the proxy will prefer to use the most recent flow that goes
to this UA instead of trying one of the old flows which would
presumably fail.
3.3. Multiple Connections from a User Agent
There are various ways to deploy SIP to build a reliable and scalable
system. This section discusses one such design that is possible with
the mechanisms in this specification. Other designs are also
possible.
In this example system, the logical proxy/registrar for the domain is
running on two hosts that share the appropriate state and can both
provide registrar and proxy functionality for the domain. The UA
will form connections to two of the physical hosts that can perform
the proxy/registrar function for the domain. Reliability is achieved
by having the UA form two TCP connections to the domain. Scalability
is achieved by using DNS SRV to load balance the primary connection
across a set of machines that can service the primary connection and
also using DNS SRV to load balance across a separate set of machines
that can service the backup connection. The deployment here requires
that DNS is configured with one entry that resolves to all the
primary hosts and another entry that resolves to all the backup
hosts. Designs having only one set were also considered, but in this
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case there would have to be some way to ensure that the two
connection did not accidentally resolve to the same host. Various
approaches for this are possible but all probably require extensions
to the SIP protocol so they were not included in this specification.
This approach can work with the disadvantage that slightly more
configuration of DNS is required.
+-------------------+
| Domain |
| Logical Proxy/Reg |
| |
|+-----+ +-----+|
||Host1| |Host2||
|+-----+ +-----+|
+---\------------/--+
\ /
\ /
\ /
\ /
+------+
| User |
| Agent|
+------+
The UA is configured with a primary and backup registration URI.
These URIs are configured into the UA through whatever the normal
mechanism is to configure the proxy or registrar address in the UA.
If the AOR is Alice@example.com, the outbound-proxy-set might look
something like "sip:primary.example.com;sip-stun" and "sip:
backup.example.com;sip-stun". The "sip-stun" tag indicates that a
SIP server supports STUN and SIP muxed over the same flow, as
described later in this specification. Note that each URI in the
outbound-proxy-set could resolve to several different physical hosts.
The administrative domain that created these URIs should ensure that
the two URIs resolve to separate hosts. These URIs are handled
according to normal SIP processing rules, so things like SRV can be
used to do load balancing across a proxy farm.
The domain also needs to ensure that a request for the UA sent to
host1 or host2 is then sent across the appropriate flow to the UA.
The domain might choose to use the Path header (as described in the
next section) approach to store this internal routing information on
host1 or host2.
When a single server fails, all the UAs that have a flow through it
will detect a flow failure and try to reconnect. This can cause
large loads on the server. When large numbers of hosts reconnect
nearly simultaneously, this is referred to as the avalanche restart
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problem, and is further discussed in Section 4.5. The multiple flows
to many servers help reduce the load caused by the avalanche restart.
If a UA has multiple flows, and one of the servers fails, it can
delay some significant time before trying to form a new connection to
replace the flow to the server that failed. By spreading out the
time used for all the UAs to reconnect to a server, the load on the
server farm is reduced.
3.4. Edge Proxies
Some SIP deployments use edge proxies such that the UA sends the
REGISTER to an Edge Proxy that then forwards the REGISTER to the
Registrar. The Edge Proxy includes a Path header [12] so that when
the registrar later forwards a request to this UA, the request is
routed through the Edge Proxy. There could be a NAT or firewall
between the UA and the Edge Proxy.
+---------+
|Registrar|
|Proxy |
+---------+
/ \
/ \
/ \
+-----+ +-----+
|Edge1| |Edge2|
+-----+ +-----+
\ /
\ /
----------------------------NAT/FW
\ /
\ /
+------+
|User |
|Agent |
+------+
These systems can use effectively the same mechanism as described in
the previous sections but need to use the Path header. When the Edge
Proxy receives a registration, it needs to create an identifier value
that is unique to this flow (and not a subsequent flow with the same
addresses) and put this identifier in the Path header URI. This can
be done by putting the value in the user portion of a loose route in
the path header. If the registration succeeds, the Edge Proxy needs
to map future requests that are routed to the identifier value from
the Path header, to the associated flow.
The term Edge Proxy is often used to refer to deployments where the
Edge Proxy is in the same administrative domain as the Registrar.
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However, in this specification we use the term to refer to any proxy
between the UA and the Registrar. For example the Edge Proxy may be
inside an enterprise that requires its use and the registrar could be
a service provider with no relationship to the enterprise.
Regardless if they are in the same administrative domain, this
specification requires that Registrars and Edge proxies support the
Path header mechanism in RFC 3327 [12].
3.5. Keep Alive Technique
A keep alive mechanism needs to detect failure of a connection and
changes to the NAT public mapping, as well as keeping any NAT
bindings refreshed. This specification uses STUN [7] over the same
flow as the SIP traffic to perform the keep alive. A flow definition
could change because a NAT device in the network path reboots and the
resulting public IP address or port mapping for the UA changes. To
detect this, requests are sent over the same flow that is being used
for the SIP traffic. The proxy or registrar acts as a STUN server on
the SIP signaling port.
Note: The STUN mechanism is very robust and allows the detection
of a changed IP address. Many other options were considered. It
may also be possible to detect a changes flow with OPTIONS
messages and the rport parameter. Although the OPTIONS approach
has the advantage of being backwards compatible, it also
significantly increases the load on the proxy or registrar server.
The TCP KEEP_ALIVE mechanism was not used because most operating
systems do not allow the time to be set on a per connection basis.
Linux, Solaris, OS X, and Windows all allow KEEP_ALIVEs to be
turned on or off on a single socket using the SO_KEEPALIVE socket
options but can not change the duration of the timer for an
individual socket. The length of the timer typically defaults to
7200 seconds. The length of the timer can be changed to a smaller
value by setting a kernel parameter but that affects all TCP
connections on the host and thus is not appropriate to use.
When the UA detects that a flow has failed or that the flow
definition has changed, the UA needs to re-register and will use the
back-off mechanism described in Section 4 to provide congestion
relief when a large number of agents simultaneously reboot.
4. User Agent Mechanisms4.1. Instance ID Creation
Each UA MUST have an Instance Identifer URN that uniquely identifies
the device. Usage of a URN provides a persistent and unique name for
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the UA instance. It also provides an easy way to guarantee
uniqueness within the AOR. This URN MUST be persitant across power
cylces of the device.
A UA SHOULD use a UUID URN [9]. The UUID URN allows for non-
centralized computation of a URN based on time, unique names (such as
a MAC address), or a random number generator.
A device like a soft-phone, when first installed, can generate a
UUID [9] and then save this in persistent storage for all future
use. For a device such as a hard phone, which will only ever have
a single SIP UA present, the UUID can include the MAC address and
be generated at any time because it is guaranteed that no other
UUID is being generated at the same time on that physical device.
This means the value of the time component of the UUID can be
arbitrarily selected to be any time less than the time when the
device was manufactured. A time of 0 (as shown in the example in
Section 3.2) is perfectly legal as long as the device knows no
other UUIDs were generated at this time.
If a URN scheme other than UUID is used, the URN MUST be selected
such that the instance can be certain that no other instance
registering against the same AOR would choose the same URN value. An
example of a URN that would not meet the requirements of this
specification is the national bibliographic number [15]. Since there
is no clear relationship between a SIP UA instance and a URN in this
namespace, there is no way a selection of a value can be performed
that guarantees that another UA instance doesn't choose the same
value.
The UA SHOULD include a "sip.instance" media feature tag as a UA
characteristic [10] in requests and responses. As described in [10],
this media feature tag will be encoded in the Contact header field as
the "+sip.instance" Contact header field parameter. The value of
this parameter MUST be a URN [3]. One case where a UA may not want
to include the URN in the sip.instance media feature tag is when it
is making an anoymous request or some other privacy concern requires
that the UA not reveal its identity.
RFC 3840 [10] defines equality rules for callee capabilities
parameters, and according to that specification, the
"sip.instance" media feature tag will be compared by case-
sensitive string comparison. This means that the URN will be
encapsulated by angle brackets ("<" and ">") when it is placed
within the quoted string value of the +sip.instance Contact header
field parameter. The case-sensitive matching rules apply only to
the generic usages defined in RFC 3840 [10] and in the caller
preferences specification [2]. When the instance ID is used in
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this specification, it is effectively "extracted" from the value
in the "sip.instance" media feature tag. Thus, equality
comparisons are performed using the rules for URN equality that
are specific to the scheme in the URN. If the element performing
the comparisons does not understand the URN scheme, it performs
the comparisons using the lexical equality rules defined in RFC2141 [3]. Lexical equality may result in two URNs being
considered unequal when they are actually equal. In this specific
usage of URNs, the only element which provides the URN is the SIP
UA instance identified by that URN. As a result, the UA instance
SHOULD provide lexically equivalent URNs in each registration it
generates. This is likely to be normal behavior in any case;
clients are not likely to modify the value of the instance ID so
that it remains functionally equivalent yet lexigraphically
different to previous registrations.
4.2. Initial Registrations
UAs are configured with one or more SIP URIs representing the default
outbound-proxy-set. The specification assumes the set is determined
via configuration but future specifications may define other
mechanisms such as using DNS to discover this set. How the UA is
configured is outside the scope of this specification. However, a UA
MUST support sets with at least two outbound proxy URIs (primary and
backup) and SHOULD support sets with up to four URIs. For each
outbound proxy URI in the set, the UA MUST send a REGISTER in the
normal way using this URI as the default outbound proxy. Forming the
route set for the request is outside the scope of this document, but
typically results in sending the REGISTER such that the topmost Route
header field contains a loose route to the outbound proxy URI. Other
issues related to outbound route construction are discussed in [20].
Registration requests, other than those described in Section 4.2.1,
MUST include the instance-id media feature tag as specified in
Section 4.1.
These ordinary registration requests MUST also add a distinct reg-id
parameter to the Contact header field. Each one of these
registrations will form a new flow from the UA to the proxy. The
reg-id sequence does not have to be sequential but MUST be exactly
the same reg-id sequence each time the device power cycles or reboots
so that the reg-id values will collide with the previously used
reg-id values. This is so the proxy can realize that the older
registrations are probably not useful.
The UAC MUST indicate that it supports the Path header [12]
mechanism, by including the 'path' option-tag in a Supported header
field value in its REGISTER requests. Other than optionally
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examining the Path vector in the response, this is all that is
required of the UAC to support Path.
The UAC MAY examine successful registrations for the presence of an
'outbound' option-tag in a Supported header field value. Presence of
this option-tag indicates that the registrar is compliant with this
specification.
Note that the UA needs to honor 503 responses to registrations as
described in RFC 3261 and RFC 3263 [6]. In particular, implementors
should note that when receiving a 503 response with a Retry-After
header field, the UA should wait the indicated amount of time and
retry the registration. A Retry-After header field value of 0 is
valid and indicates the UA should retry the REGISTER immediately.
Implementations need to ensure that when retrying the REGISTER they
revisit the DNS resolution results such that the UA can select an
alternate host from the one chosen the previous time the URI was
resolved.
4.2.1. Registration by Other Instances
A User Agent MUST NOT include an instance-id or reg-id in the Contact
header field of a registration if the registering UA is not the same
instance as the UA referred to by the target Contact header field.
(This practice is occasionally used to install forwarding policy into
registrars.)
Note that a UAC also MUST NOT include an instance-id or reg-id
parameter in a request to deregister all Contacts (a single Contact
header field value with the value of "*").
4.3. Sending Requests
As described in Section 4.1, all requests need to include the
instance-id media feature tag unless privacy concerns require
otherwise.
4.3.1. Selecting the First Hop
When an UA is about to send a request, it first performs normal
processing to select the next hop URI. The UA can use a variety of
techniques to compute the route set and accordingly the next hop URI.
Discussion of these techniques is outside the scope of this document
but could include mechanisms specified in RFC 3608 [21] (Service
Route) and [20].
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The UA performs normal DNS resolution on the next hop URI (as
described in RFC 3263 [6]) to find a protocol, IP address, and port.
For non TLS protocols, if the UA has an existing flow to this IP
address, and port with the correct protocol, then the UA MUST use the
existing connection. For TLS protocols, the existing flow is only
used if, in addition to matching the IP address, port, and protocol,
the host production in the next hop URI MUST match one of the URIs
contained in the subjectAltName in the peer certificate. If the UA
cannot use one of the existing flows, then it SHOULD form a new flow
by sending a datagram or opening a new connection to the next hop, as
appropriate for the transport protocol.
4.4. Detecting Flow Failure
The UA needs to detect when a specific flow fails. If a flow has
failed, the UA follows the procedures in Section 4.2 to form a new
flow to replace the failed one. The UA proactively tries to detect
failure by periodically sending keep alive messages using one of the
techniques described in this section.
The time between keep alive requests when using UDP based transports
SHOULD be a random number between 24 and 29 seconds while for TCP
based transports it SHOULD be a random number between 95 and 120
seconds. These times MAY be configurable.
o Note on selection of time values: For UDP, the upper bound of 29
seconds was selected so that multiple STUN packets could be sent
before 30 seconds based on information that many NATs have UDP
timeouts as low as 30 seconds. The 24 second lower bound was
selected so that after 10 minutes the jitter introduced by
different timers will the keep alive requests unsynchronized to
evenly spread the load on the servers. For TCP, the 120 seconds
was chosen based on the idea that for a good user experience,
failures should be detected in this amount of time and a new
connection set up. Operators that wish to change the relationship
between load on servers and the expected time that a user may not
receive inbound communications will probably adjust this time.
The 95 seconds lower bound was chosen so that the jitter
introduced will result in a relatively even load on the servers
after 30 minutes.
4.4.1. Keep Alive with STUN
User Agents that form flows MUST check if the configured URI they are
connecting to has the "sip-stun" URI parameter (defined in
Section 10). If the parameter is present, the UA needs to
periodically perform keep alive checks by sending a STUN [7] Binding
Requests over the flow.
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If the XOR-MAPPED-ADDRESS in the STUN Binding Response changes, the
UA MUST treat this event as a failure on the flow.
4.4.2. Keep Alive with Double CRLF
User Agents that form flows MUST check if the configured URI they are
connecting to has the "crlf-ping" URI parameter (defined in
Section 10). If the parameter is present, the UA needs to send keep
alive requests by sending a CRLF over the flow.
If the UA does not receive any data back over the flow within 7
seconds of sending the CRLF, then it MUST consider the lack of
response to be a flow failure.
4.5. Flow Recovery
When a flow to a particular URI in the outbound-proxy-set fails, the
UA needs to form a new flow to replace the old flow and replace any
registrations that were previously sent over this flow. Each new
registration MUST have the same reg-id as the registration it
replaces. This is done in much the same way as forming a brand new
flow as described in Section 4.3.2; however, if there is a failure in
forming this flow, the UA needs to wait a certain amount of time
before retrying to form a flow to this particular next hop.
The time to wait is computed in the following way. If all of the
flows to every URI in the proxy set have failed, the base time is set
to 30 seconds; otherwise, in the case where at least one of the flows
has not failed, the base time is set to 90 seconds. The wait time is
computed by taking two raised to power of the number of consecutive
registration failures for that URI, and multiplying this by the base
time, up to a maximum of 1800 seconds.
wait-time = min( 1800, (base-time * (2 ^ consecutive-failures)))
These three times MAY be configurable in the UA. The three times are
the max-time with a default of 1800 seconds, the base-time-all-fail
with a default of 30 seconds, and the base-time-not-failed with a
default of 60 seconds. For example if the base time was 30 seconds,
and there had been three failures, then the wait time would be
min(1800,30*(2^3)) or 240 seconds. The delay time is computed by
selecting a uniform random time between 50 and 100 percent of the
wait time. The UA MUST wait for the value of the delay time before
trying another registration to form a new flow for that URI.
To be explicitly clear on the boundary conditions: when the UA boots
it immediately tries to register. If this fails and no registration
on other flows succeed, the first retry happens somewhere between 30
and 60 seconds after the failure of the first registration request.
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If the number of consecutive-failures is large enough that the
maximum of 1800 seconds is reached, the UA will keep trying forever
with a random time between 900 and 1800 seconds between the attempts.
5. Edge Proxy Mechanisms5.1. Processing Register Requests
When an Edge Proxy receives a registration request with a
sip.instance media feature tag in the Contact header field, it MUST
form a flow identifier token that is unique to this network flow.
The Edge Proxy MUST insert this token into a URI referring to this
proxy and place this URI into a Path header field as described in RFC3327 [12]. The token MAY be placed in the userpart of the URI.
5.2. Generating Flow Tokens
A trivial but impractical way to satisfy the flow token requirement
Section 5.1 involves storing a mapping between an incrementing
counter and the connection information; however this would require
the Edge Proxy to keep an impractical amount of state. It is unclear
when this state could be removed and the approach would have problems
if the proxy crashed and lost the value of the counter. Two
stateless examples are provided below. A proxy can use any algorithm
it wants as long as the flow token is unique to a flow, the flow can
be recovered from the token, and the token can not be modified by
attackers.
Algorithm 1: The proxy generates a flow token for connection-oriented
transports by concatenating the file descriptor (or equivalent)
with the NTP time the connection was created, and base64 encoding
the result. This results in an approximately 16 octet identifier.
The proxy generates a flow token for UDP by concatenating the file
descriptor and the remote IP address and port, then base64
encoding the result. This algorithm MUST NOT be used unless all
messages between the Edge proxy and Registrar use a SIPS protected
transport. If the SIPS level of integrity protection is not
available, an attacker can hijack another user's calls.
Algorithm 2: When the proxy boots it selects a 20 byte crypto random
key called K that only the Edge Proxy knows. A byte array, called
S, is formed that contains the following information about the
flow the request was received on: an enumeration indicating the
protocol, the local IP address and port, the remote IP address and
port. The HMAC of S is computed using the key K and the HMAC-
SHA1-80 algorithm, as defined in [16]. The concatenation of the
HMAC and S are base64 encoded, as defined in [18], and used as the
flow identifier. When using IPv4 addresses, this will result in a
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32 octet identifier.
5.3. Forwarding Requests
When the Edge Proxy receives a request, it applies normal routing
procedures with the following addition. If the top-most Route header
refers to the Edge Proxy and contains a valid flow identifier token
created by this proxy, the proxy MUST forward the request over the
flow that received the REGISTER request that caused the flow
identifier token to be created. For connection-oriented transports,
if the flow no longer exists the proxy SHOULD send a 410 response to
the request.
The advantage to a stateless approach to managing the flow
information is that there is no state on the edge proxy that
requires clean up or that has to be synchronized with the
registrar.
Proxies which used one of the two algorithms described in this
document to form a flow token follow the procedures below to
determine the correct flow.
Algorithm 1: The proxy base64 decodes the user part of the Route
header. For TCP, if a connection specified by the file descriptor
is present and the creation time of the file descriptor matches
the creation time encoded in the Route header, the proxy forwards
the request over that connection. For UDP, the proxy forwards the
request from the encoded file descriptor to the source IP address
and port.
Algorithm 2: To decode the flow token take the flow identifier in the
user portion of the URI, and base64 decode it, then verify the
HMAC is correct by recomputing the HMAC and checking it matches.
If the HMAC is not correct, the proxy SHOULD send a 403 response.
If the HMAC was correct then the proxy should forward the request
on the flow that was specified by the information in the flow
identifier. If this flow no longer exists, the proxy SHOULD send
a 410 response to the request.
Note that techniques to ensure that mid-dialog requests are routed
over an existing flow are out of scope and therefore not part of this
specification. However, an approach such as having the Edge Proxy
Record-Route with a flow token is one way to ensure that mid-dialog
requests are routed over the correct flow.
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Internet-Draft Client Initiated Connections in SIP March 20066.1. Processing Register Requests
This specification updates the definition of a binding in RFC 3261
[5] Section 10 and RFC 3327 [12] Section 5.3.
When no instance-id is present in a Contact header field value in a
REGISTER request, the corresponding binding is still between an AOR
and the URI from that Contact header field value. When an
instance-id is present in a Contact header field value in a REGISTER
request, the corresponding binding is between an AOR and the
combination of instance-id and reg-id. For a binding with an
instance-id, the registrar still stores the Contact header field
value URI with the binding, but does not consider the Contact URI for
comparison purposes (the Contact URI is not part of the "key" for the
binding). The registrar MUST be prepared to receive, simultaneously
for the same AOR, some registrations that use instance-id and reg-id
and some that do not.
Registrars which implement this specification, MUST support the Path
header mechanism [12].
In addition to the normal information stored in the binding record,
some additional information MUST be stored for any registration that
contains a reg-id header parameter in the Contact header field value.
The registrar MUST store enough information to uniquely identify the
network flow over which the request arrived. For common operating
systems with TCP, this would typically just be the file descriptor.
For common operating systems with UDP this would typically be the
file descriptor for the local socket that received the request, the
local interface, and the IP address and port number of the remote
side that sent the request.
The registrar MUST also store all the Contact header field
information including the reg-id and instance-id parameters and
SHOULD also store the time at which the binding was last updated. If
a Path header field is present, RFC 3327 [12] requires the registrar
to store this information as well. If the registrar receives a re-
registration, it MUST update the information that uniquely identifies
the network flow over which the request arrived and SHOULD update the
time the binding was last updated.
The Registrar MUST include the 'outbound' option-tag in a Supported
header field value in its responses to REGISTER requests. The
Registrar MAY be configured with local policy to reject any
registrations that do not include the instance-id and reg-id to
eliminate the amplification attack described in [19]. Note that the
requirements in this section applies to both REGISTER requests
received from an Edge Proxy as well as requests received directly
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from the UAC.
6.2. Forwarding Requests
When a proxy uses the location service to look up a registration
binding and then proxies a request to a particular contact, it
selects a contact to use normally, with a few additional rules:
o The proxy MUST NOT populate the target set with more than one
contact with the same AOR and instance-id at a time. If a request
for a particular AOR and instance-id fails with a 410 response,
the proxy SHOULD replace the failed branch with another target (if
one is available) with the same AOR and instance-id, but a
different reg-id.
o If two bindings have the same instance-id and reg-id, the proxy
SHOULD prefer the contact that was most recently updated.
The proxy uses normal forwarding rules looking at the Route of the
message and the value of any stored Path header field vector in the
registration binding to decide how to forward the request and
populate the Route header in the request. Additionally, when the
proxy forwards a request to a binding that contains a reg-id, the
proxy MUST send the request over the same network flow that was saved
with the binding. This means that for TCP, the request MUST be sent
on the same TCP socket that received the REGISTER request. For UDP,
the request MUST be sent from the same local IP address and port over
which the registration was received, to the same IP address and port
from which the REGISTER was received.
If a proxy or registrar receives information from the network that
indicates that no future messages will be delivered on a specific
flow, then the proxy MUST invalidate all the bindings that use that
flow (regardless of AOR). Examples of this are a TCP socket closing
or receiving a destination unreachable ICMP error on a UDP flow.
Similarly, if a proxy closes a file descriptor, it MUST invalidate
all the bindings with flows that use that file descriptor.
7. Mechanisms for All Servers (Proxys, Registars, UAS)
A SIP device that receives SIP messages directly from a UA needs to
behave as specified in this section. Such devices would generally
include a Registrar and an Edge Proxy, as they both receive register
requests directly from a UA.
7.1. STUN Processing
This document defines a new STUN usage for inband connectivity
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checks. The only STUN messages required by this usage are Binding
Requests, Binding Responses, and Error Responses. The UAC sends
Binding Requests over the same UDP flow, TCP connection, or TLS
channel used for sending SIP messages, once a SIP registration has
been successfully processed on that flow. These Binding Requests do
not require any STUN attributes. The UAS responds to a valid Binding
Request with a Binding Response which MUST include the XOR-MAPPED-
ADDRESS attribute. After a successful STUN response is received over
TCP or TLS over TCP, the underlying TCP connection is left in the
active state.
If the server receives SIP requests on a given interface and port, it
MUST also provide a limited version of a STUN server on the same
interface and port. Specifically it MUST be capable of receiving and
responding to STUN Binding Requests.
It is easy to distinguish STUN and SIP packets because the first
octet of a STUN packet has a value of 0 or 1 while the first octet
of a SIP message is never a 0 or 1.
When a URI is created that refers to a SIP device that supports STUN
as described in this section, the URI parameter "sip-stun", as
defined in Section 10 MUST be added to the URI. This allows a UA to
inspect the URI to decide if it should attempt to send STUN requests
to this location. The sip-stun tag typically would be present in the
URI in the Route header field value of a REGISTER request and not be
in the Request URI.
7.2. Double CRLF Processing
If the SIP server is acting as the TCP client and initiated the TCP
connection (meaning that this host did the active open), then the SIP
server MUST NOT perform any of the processing in this section. The
following only applies when the SIP server is acting as the TCP
server (meaning that this host did the passive open).
When the server receives a CRLF before the start line of a message on
a flow, it MUST send some data back on that same flow within 3
seconds. If no message is actively being sent, it SHOULD send back a
CRLF after waiting at least 1 second. The reason for waiting at
least 1 second is that if the other end has an incorrect
implementation and incorrectly echoes the CRLF, this will stop the
flow from going into a live-lock state.
8. Example Message Flow
The following call flow shows a basic registration and an incoming
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;reg-id=2
Content-Length: 0
SIP/2.0 200 OK
Via: SIP/2.0/UDP 10.0.1.1;branch=z9hG4bKnashds7
From: Callee <sip:callee@example.com>;tag=a73kszlfl
To: Callee <sip:callee@example.com> ;tag=b88sn
Call-ID: 1j9FpLxk3uxtm8tn-2@10.0.1.1
Supported: outbound
CSeq: 1 REGISTER
Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;reg-id=1
;expires=3600
Contact: <sip:callee@10.0.1.1>
;+sip.instance="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>"
;reg-id=2
;expires=3600
Content-Length: 0
The messages in the call flow are very normal. The only interesting
thing to note is that the INVITE in message 6 contains the following
Record-Route header field:
Record-Route: <sip:example.com;lr>
Message 11 seems seams strange in that it goes to the backup instead
of the primary. The Caller actually sends the message to the domain
of the callee to a host (primary or backup) that is currently
available. How the domain does this is an implementation detail up
to the domain and not part of this specification.
The registrations in message 15 and 16 are the same as message 1 and
2 other than the Call-ID has changed.
9. Grammar
This specification defines new Contact header field parameters,
reg-id and +sip.instance. The grammar includes the definitions from
RFC 3261 [5] and includes the definition of uric from RFC 2396 [11].
The ABNF[8] is:
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contact-params = c-p-q / c-p-expires / c-p-flow / c-p-instance
/ contact-extension
c-p-flow = "reg-id" EQUAL 1*DIGIT ; 1 to 2**31
c-p-instance = "+sip.instance" EQUAL LDQUOT "<"
instance-val ">" RDQUOT
instance-val = *uric ; defined in RFC 2396
The value of the reg-id MUST NOT be 0 and MUST be less than 2**31.
10. IANA Considerations10.1. Contact Header Field
This specification defines a new Contact header field parameter
called reg-id in the "Header Field Parameters and Parameter Values"
sub-registry as per the registry created by [13] . The required
information is:
Header Field Parameter Name Predefined Reference
Values
____________________________________________________________________
Contact reg-id Yes [RFC AAAA]
[NOTE TO RFC Editor: Please replace AAAA with
the RFC number of this specification.]
10.2. SIP/SIPS URI Paramters
This specification arguments the "SIP/SIPS URI Parameters" sub-
registry as per the registry created by [14] . The required
information is:
Parameter Name Predefined Values Reference
____________________________________________
sip-stun No [RFC AAAA]
crlf-ping No [RFC AAAA]
[NOTE TO RFC Editor: Please replace AAAA with
the RFC number of this specification.]
10.3. SIP Option Tag
This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of RFC 3261.
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Name: outbound
Description: This option-tag is used to identify Registrars which
support extensions for Client Initiated Connections. A Registrar
places this option-tag in a Supported header to communicate to the
registering User Agent the Registrars support for this extension.
10.4. Media Feature Tag
This section registers a new media feature tag, per the procedures
defined in RFC 2506 [1]. The tag is placed into the sip tree, which
is defined in RFC 3840 [10].
Media feature tag name: sip.instance
ASN.1 Identifier: New assignment by IANA.
Summary of the media feature indicated by this tag: This feature tag
contains a string containing a URN that indicates a unique identifier
associated with the UA instance registering the Contact.
Values appropriate for use with this feature tag: String.
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms: This
feature tag is most useful in a communications application, for
describing the capabilities of a device, such as a phone or PDA.
Examples of typical use: Routing a call to a specific device.
Related standards or documents: RFC XXXX
[[Note to IANA: Please replace XXXX with the RFC number of this
specification.]]
Security Considerations: This media feature tag can be used in ways
which affect application behaviors. For example, the SIP caller
preferences extension [23] allows for call routing decisions to be
based on the values of these parameters. Therefore, if an attacker
can modify the values of this tag, they may be able to affect the
behavior of applications. As a result, applications which utilize
this media feature tag SHOULD provide a means for ensuring its
integrity. Similarly, this feature tag should only be trusted as
valid when it comes from the user or user agent described by the tag.
As a result, protocols for conveying this feature tag SHOULD provide
a mechanism for guaranteeing authenticity.
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Internet-Draft Client Initiated Connections in SIP March 200611. Security Considerations
One of the key security concerns in this work is making sure that an
attacker cannot hijack the sessions of a valid user and cause all
calls destined to that user to be sent to the attacker.
The simple case is when there are no edge proxies. In this case, the
only time an entry can be added to the routing for a given AOR is
when the registration succeeds. SIP already protects against
attackers being able to successfully register, and this scheme relies
on that security. Some implementers have considered the idea of just
saving the instance-id without relating it to the AOR with which it
registered. This idea will not work because an attacker's UA can
impersonate a valid user's instance-id and hijack that user's calls.
The more complex case involves one or more edge proxies. When a UA
sends a REGISTER request through an Edge Proxy on to the registrar,
the Edge Proxy inserts a Path header field value. If the
registration is successfully authenticated, the proxy stores the
value of the Path header field. Later when the registrar forwards a
request destined for the UA, it copies the stored value of the Path
header field into the route header field of the request and forwards
the request to the Edge Proxy.
The only time an Edge Proxy will route over a particular flow is when
it has received a route header that has the flow identifier
information that it has created. An incoming request would have
gotten this information from the registrar. The registrar will only
save this information for a given AOR if the registration for the AOR
has been successful; and the registration will only be successful if
the UA can correctly authenticate. Even if an attacker has spoofed
some bad information in the path header sent to the registrar, the
attacker will not be able to get the registrar to accept this
information for an AOR that does not belong to the attacker. The
registrar will not hand out this bad information to others, and
others will not be misled into contacting the attacker.
12. Open Issues
Do we want to include the Double CRLF keep alive option?
Are thre any deployments that could use Algorithm 1 and if not can we
remove it?
We should change syntax from "sip-stun" to "keep-alive=sip-stun".
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Internet-Draft Client Initiated Connections in SIP March 200613. Requirements
This specification was developed to meet the following requirements:
1. Must be able to detect that a UA supports these mechanisms.
2. Support UAs behind NATs.
3. Support TLS to a UA without a stable DNS name or IP address.
4. Detect failure of connection and be able to correct for this.
5. Support many UAs simultaneously rebooting.
6. Support a NAT rebooting or resetting.
7. Minimize initial startup load on a proxy.
8. Support architectures with edge proxies.
14. Changes
Note to RFC Editor: Please remove this whole section.
14.1. Changes from 01 Version
Moved definition of instance-id from GRUU[17] draft to this draft.
Added tentative text about Double CRLF Keep Alive
Removed pin-route stuff
Changed the name of "flow-id" to "reg-id"
Reorganized document flow
Described the use of STUN as a proper STUN usage
Added 'outbound' option-tag to detect if registrar supports outbound
14.2. Changes from 00 Version
Moved TCP keep alive to be STUN.
Allowed SUBSCRIBE to create flow mappings. Added pin-route option
tags to support this.
Added text about updating dialog state on each usage after a
connection failure.
15. Acknowledgments
Jonathan Rosenberg provided many comments and useful text. Dave Oran
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